1. Field of the Invention
This invention relates to a color isolation method, for a picture reproducing machine such as a color scanner for making printing plates, wherein portions of an original picture corresponding to a certain specific color is selectively isolated and in particular to a method wherein a specific color is given in advance and portions of an original picture having a color corresponding to the given color are isolated simultaneously as the original picture is scanned. Such a process is useful for compensating or totally changing a certain selected color of an original picture without changing the colors of the remaining part of the original picture.
2. Description of the Prior Art
According to a conventional method, such a process is accomplished by comparing the ratios between three basic colors in the picture signals obtained by scanning an original picture. In terms of a color plane in which three basic colors are represented by three coordinate axes extending from an origin 120 degrees apart from one another so that each color may be represented by polar coordinates, the angle corresponding to the hue of the color while the radius corresponding to the color saturation of the color, one can identify the color as to which one of the three sectors defined by the three coordinate axes the color belongs to by comparing the ratios between the components of the three basic colors at each point of the picture as it is scanned. For example, if a certain color is composed of red, green and blue components and the red component is the minimum of the three components, then the color may be identified to the extent that it lies in the sector defined by the axes corresponding to green and blue colors. Such a process can be easily implemented using a strictly analog process. However, the output will assume only two values, yes or no, or, in another words, whether the colors are in the same sector of the color plane or not.
It is also known that, by adding another three coordinate axes corresponding to the three colors complementary to red, green and blue to the color plane and defining six sectors in the color plane, an improvement of the accuracy in the color identification over the above described method can be effected.
However, with either one of the above described conventional methods, the resolution in color isolation is not sufficient for distinguishing two colors having a small color tone difference therebetween. Therefore, they are not useful for most applications.